A field of the invention is material testing of microscale and nanoscale films.
As part of micro-electronic and micro-electro-mechanical systems (MEMS), thin films experience extrinsic loads due to operational and environmental conditions of the devices, and may fail to maintain mechanical integrity, as observed by cracking, delamination, and void or hillock formation under stresses. Accurate prediction of thin film material response requires understanding of the fundamental mechanisms of material deformation and fracture occurrence in the microscale and nanoscale. Material properties typically cannot be extrapolated from their respective bulk values since material behavior often is not only different in the microscale, but is also significantly affected by fabrication processes, and is very sensitive to the influences of interfaces and adjoining materials.
Various material properties tests require tensile loading of a specimen. For example, one popular testing method for both bulk and thin film materials is the uniaxial tensile test. This basically involves gripping a specimen at opposing ends, and producing tension in the specimen along a single axis while measuring the specimen""s response. When loading thin film materials, however, certain challenges are involved, such as: fabricating a freestanding specimen with minimal pre-stress; gripping of the specimen; aligning of the specimen in the direction of the force (to minimize likelihood of invalidation of the test caused by flexural stress on the specimen and resultant premature failure); and generating small forces (on the order of micro-Newtons) with high resolutions.
These challenges can be addressed by using a substrate layer that is usually very compliant and with known material properties along with the actual film to be tested. However, introduction of the substrate complicates the experimental analysis because the microscale material properties of the substrate itself may not be known accurately, and because the interface with the substrate may influence the mechanical behavior of the film.
For example, a method of fabricating freestanding aluminum films includes evaporating metal film on a glass slide covered with a water-soluble layer, releasing the thin film from the glass slide by immersing it in water, and gluing the film to grips of a nano-tensilometer with epoxy. However, problems of mounting the specimen and premature specimen failure invalidate a significant number of tests using this method, and experimental results from the tests have shown significant variation in measured elastic modulus and ultimate tensile strength.
Another prior fabrication technique includes sputtering metal films on glass slides and releasing the films by peeling the films off from a substrate. A motor-driven micrometer is used to produce elongation in the films, and a load cell is used to read the stress. Laser spots diffracted from the gratings on the specimen surface determines the strain with 0.002% resolution.
Another known method produces a piezo-actuated tensile testing apparatus using Tixe2x80x94Cuxe2x80x94Ti multilayer films with a length of 700 microns, a width of 200 microns, and a total thickness of 1.2 xcexcm patterned on wafers by lithography. The films are then released from the substrate by wet etching of the substrate. This tensile testing apparatus has been known to provide force and displacement resolutions of about 200 xcexcN and 20 nm, respectively. Still another testing method uses piezoelectric actuators for displacement and a load cell, a laser interferometer, and a strain gauge-optical encoder assembly to measure force and displacement.
The present invention provides a method and apparatus for testing of a thin-film specimen. A chip includes a free-spanning specimen to be tested. The specimen is co-fabricated with the remainder of the chip and is aligned with a longitudinal direction of the chip, along a tensile axis. The chip includes a first end, a second end, and one or more side beams. A force sensor beam supports a first longitudinal end of the specimen, and a support structure supports a second longitudinal end of the specimen.
The support structure may include a longitudinal beam connected to the second longitudinal end of the specimen and aligned with the tensile axis of the specimen. Additionally, the support structure may include a plurality of support beams to reduce undesirable flexing of the specimen resulting from misalignment of the pulling direction with the tensile axis of the specimen. Preferably, the chip includes a pair of structural springs fabricated for maintaining structural integrity between the first and second ends of the chip. The chip may additionally include markers to measure displacement of longitudinal ends of the specimen and deflection of the force sensor beam.
In a preferred method of testing a thin film specimen, a chip is provided that is co-fabricated with the specimen, and first and second ends of the chip are mounted to first and second sections, respectively, of a straining stage within an environmental chamber. The straining stage is actuated to move the first section of the straining stage away from the second section, thus pulling the first and second ends of the chip along a pulling or displacement direction and straining the gauge length of the specimen generally along the tensile axis.